Use pstree inside awk command

Question

I want to run pstree on a set of pid which I find using ps

ps -aux | grep ^username | awk '{pstree $2}'

Unfortunately, the output is empty, but if I run the pstree command manually with the same pids I get the desired output. What is wrong with the awk command? Or how do I achieve the desired result by other means?

As an aside, `man ps` would tell you: Note that __"ps -aux"__ is distinct from __"ps aux"__. — devnull, Dec 06 '13 at 14:04
`ps -a -u username -x -o pid` should return the same output as the given pipeline (at least for BSD `ps`; the program varies greatly from platform to platform). — chepner, Dec 06 '13 at 14:15
What happens if you run pstree inside a C program? I'd imagine the result would be identical to that wouldn't it since neither awk nor C are shell? — Ed Morton, Dec 06 '13 at 15:25

score 3 · Answer 1 · answered Dec 06 '13 at 14:02

3

Try

  ps -aux | grep ^username | awk '{print $2}' | xargs pstree

(As is, pstree is an empty variable value )

This can can be boiled down to

ps -aux |  awk '/^username/{print $2}' | xargs pstree

IHTH

answered Dec 06 '13 at 14:02

shellter

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+1 for separating the call to `pstree` from the call to `awk` (which prompted my comment on using `ps` alone to get the desired PIDs). – chepner Dec 06 '13 at 15:13
There you go - an actual UNIX solution with every command doing what it's intended for and is good at! – Ed Morton Dec 06 '13 at 15:28
High praise from 2 gurus, :-! Thanks and Good luck to all! – shellter Dec 06 '13 at 17:44

score 2 · Answer 2 · answered Dec 06 '13 at 14:02

2

use the system function in awk. Also you dont need grep here too

ps -aux | awk '$1=="username"{system("pstree $2")}'

answered Dec 06 '13 at 14:02

Shiplu Mokaddim

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anubhava · Accepted Answer · 2013-12-06T14:13:27.583

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Use system function in awk like this:

awk '{system("pstree " $2)}'

You can shorten your command to:

ps -aux | awk ' /^username/ { system("pstree " $2) }'

edited Dec 06 '13 at 14:13

answered Dec 06 '13 at 14:02

anubhava

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Great! this is what worked for me `ps -aux | grep ^username | awk '{system("pstree "$2)}'` – e271p314 Dec 06 '13 at 14:11
@e271p314: You can also shorten your command as in my edited answer. – anubhava Dec 06 '13 at 14:13

Use pstree inside awk command

3 Answers3